*Article* **Investigating the Effect of Heterogeneous Hull Roughness on Ship Resistance Using CFD**

**Soonseok Song 1, Yigit Kemal Demirel 1,\*, Claire De Marco Muscat-Fenech 2, Tonio Sant 2, Diego Villa 3, Tahsin Tezdogan 1 and Atilla Incecik 4**

**Abstract:** Research into the effects of hull roughness on ship resistance and propulsion is well established, however, the effect of heterogeneous hull roughness is not yet fully understood. In this study, Computational Fluid Dynamics (CFD) simulations were conducted to investigate the effect of heterogeneous hull roughness on ship resistance. The Wigley hull was modelled with various hull conditions, including homogeneous and heterogeneous hull conditions. The results were compared against existing experimental data and showed a good agreement, suggesting that the CFD approach is valid for predicting the effect of heterogeneous hull roughness on ship resistance. Furthermore, the local distributions of the wall shear stress and roughness Reynolds number on the hull surface were examined to assess the flow characteristics over the heterogeneous hull roughness.

**Keywords:** roughness effect; Wigley hull; ship resistance; heterogeneous hull roughness; computational fluid dynamics (CFD)


**Author details**

Rossitah Selamat1 , Ismail Abustan1 \*, Mohd Rizal Arshad2 and Nurul Hana Mokhtar Kamal1

1 Faculty of Civil Engineering, Universiti Sains Malaysia, Pulau Pinang, Malaysia

2 Faculty of Electrical Engineering, Universiti Sains Malaysia, Pulau Pinang, Malaysia

\*Address all correspondence to<EMAIL_ADDRESS>
© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.


**Rainfall Pattern Analysis over India in Relation to the State of Kerala State of Kerala**

**Rainfall Pattern Analysis over India in Relation to the**

DOI: 10.5772/intechopen.72870

Shajimon K John Additional information is available at the end of the chapter

Shajimon K John

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.72870

#### **Abstract**

Rainfall over a region across the earth is the prima face reason of its uniqueness. Different regions over the earth having different rainfall pattern in turn is directly related to the regions uniqueness, it is referred as ecosystem. India is one of the ancient regions having different rainfall pattern which has developed into places with its own unique ecosystem prevailing over its different states. Kerala the southernmost tip of this sub-continent has almost clear green top throughout the year in comparison to various other states in the country. This work mainly concentrates on the variation of rainfall over the state of Kerala in comparison with other states which are its neighbours. Rainfall data over the last century is being analysed for finding out whether the rainfall pattern over the state of Kerala has any relation with the rainfall pattern across the country. Solar activity and the number of sun spots and how these activities effect the rainfall pattern also a major area to be investigated.

**Keywords:** rainfall, monsoon, solar activity, Kerala, India

#### **1. Introduction**

Rain is the most beautiful activity in nature which supports the healthy living. Rain is defined as the liquid water in the form of droplets that have condensed from atmospheric water vapour and becomes heavy enough to fall under gravity in every scientific write-up. It is the most significant means of keeping the fresh water level in the earth to its normal level. Rain occurs when the water droplets or moisture are moving across the multi-dimensional temperature moisture profile [1]. We, as human being should be very much indebted to this natural phenomenon. Unfortunately, over the years due to many stated and unstated reasons the rainfall pattern has a change in its pattern as well as the amount of rainfall. Rainfall occurs when moisture contained air rises above the earth's surface and cools to form clouds which in

Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons

turn precipitate to fall as rain. This precipitation can reach earth's surface as rain, snow, sleet or hail depends on the pressure variations across the region [2, 3]. Maximum rainfall occurs near the earth's equator because these regions having very high direct sunlight impact with very high temperature which creates high level of vertical uplift or air. The presence of mountains in this region increases the uplift of air. The opposite happens in Polar Regions, where the air is very cold and contains high amount of water level which enable only low level of vertical uplift. This forces the air to descend down preventing from any cloud formation.

lot. The Himalayas act as a barrier against katabatic winds which blow from Central Asia [8–10]. Overall the country's climate inclined more towards tropical since the Tropic of Cancer passes through the middle of the country. The tropical wet climatic regions within the country can be sub divided into; tropical monsoon climate and savannah climate. The major areas which fall in tropical monsoon region are Lakshadweep, Andaman and Nicobar Islands, southern Assam, the Western Ghats and Malabar Coast. These areas experience moderate to high temperature along with high rainfall during monsoon season; from June to November. The high amount of rain received in these regions keeps these regions green throughout the year [11, 12]. The savannah climate is the most commonly prevailing climate in the country. Almost all inland areas experience the savannah (tropical wet and dry) climate, with extremely high temperature during summer and moderate to heavy rainfall during monsoon which normally occurs during June to

Rainfall Pattern Analysis over India in Relation to the State of Kerala

http://dx.doi.org/10.5772/intechopen.72870

19

The tropical dry climate is again sub divided into three mainly; sub-tropical arid (steppe) climate, tropical semi-arid (steppe) climate, and sub-tropical arid (desert) climate. Regions around Karnataka, Central Maharashtra, Andra Pradesh and some parts of Tamil Nadu experiences Tropical semi-arid climate experience with hot and dry summers during March to May and almost very unreliable and erratic rainfall. Western Rajasthan regions of Thar Desert fall into the sub-tropical arid (desert) climatic region. The region where the desert extends like parts of Punjab, and Haryana comes under sub-tropical semi-arid climate regions. These regions experience high temperature during summer and rain normally occurs during summer monsoon only. Sub-tropical humid climate experienced in most of the North and Northeast regions. In these regions summers are hot to very hot while during winter season the temperature may dip to 0°C [15, 16]. These regions also experience rainfall during summer with sporadic rain or snowfall during winter. The summer occurs during May to July while the winter occurs during December and January. The mountain climate mainly happens in the Himalayas region

with an average temperature fall of almost 0.5°C with every 100 meters rise in height.

The effect of these different climatic styles on the country is that it has mainly four different seasons; winter, summer, rainy season and autumn. The months of December, January and February are generally considered as winter months in most of the north and north eastern regions. The average temperature may vary around 10° to 15°C in the northern region while the southern regions has an average temperature between 20° to 23° centigrade [17, 18]. Some parts like western Himalayas and Kerala experiences rain during this season also. The pre monsoon season or summer or hot weather conditions occur during March to June with an average temperature rises up 35° centigrade. The central and northern and north western regions near to Thar Desert the temperature may reach up to 45° to 48°C. While the coastal areas record moderate temperature due to the influence of sea breeze. Some parts of north eastern and eastern areas witness thunderstorms also during these months. The areas near to Thar Desert witness hot and dry winds along with sand/dust storms. Around 75% of the total rain received across the country falls during the South West monsoon [19, 20]. The start of South West monsoon happens during last week of May or the first week of June and this season extends up to September. The extent of this season varies from region to region as the southern state of Kerala almost four to four and half months under this season while the western state of Rajasthan having less than two months. The monsoon arrives in the Indian

September [13, 14].

Rainfall study is mainly known as climatology which includes process of measuring, understanding, and predicting rainfall pattern across the earth surface [4, 5]. The major factors which have direct impact on the amount of rain received over a region are humidity, air pressure, region's topography and the type of cloud formed over the region. Based on the amount of rain received over a specific period is used an instrument to classify the climatic regions over the earth known as Köppen classification [4–6]. According to Köppen classification, five primary types named as tropical, dry, mild mid latitude, cold mid latitude and polar.


**2. Synergistic effect**

The rate of erosion-corrosion is higher than the sum of the rates of pure erosion and pure corrosion separately. The additional rate of material loss is caused by the interaction between corrosion and erosion. Many researchers have conducted extensive studies based on this interaction. Matsumura [9] reported that in a NaOH slurry, the loss of pure iron in the presence of corrosion flow was higher than that without corrosion flow, although the corrosion flow could be reduced to a very small extent. It was a measurement. Li and his colleagues [10] investigated the loss of aluminum in an acidic slurry containing 0.5 M NaCl and found that 40–50% of the total weight loss is related to the synergistic effect. Yui and his colleagues [11] reported that up to 86% of the total weight loss of white iron and chromium (chromate) in a slurry with low pH can indicate the synergistic effect. Zhang et al. [12] found that the weight loss due to the synergistic effect can be more than 92.1% for X60 tube steel, 94.6% for 321 stainless steel, and even 99.8% for 316 L stainless steel. Therefore, the important effect of the "synergistic effect" should be considered according to the above studies.

Although the erosion-corrosion map was made by Lim and Ashby [13], it can only quantitatively determine the erosion rate, corrosion rate, and ratio between them. It cannot explain how erosion and corrosion interact with each other or the mechanism of their effect. Many efforts have been made to investigate the mechanism of erosioncorrosion, but due to its specific complexity, it has not yet been fully determined. Researchers mainly divide the "synergistic effect" into two groups: erosion affecting corrosion and corrosion affecting erosion or corrosion increased by erosion and vice versa.

#### **2.1 The effect of erosion on corrosion**

As mentioned above, the effects of the erosion process include fluidity, corrosive environment, and particle impact. In a passive system, these effects lead to surface roughness, increased mass transfer acceleration, passive film failure, and increased localized corrosion. In an active system, these can lead to all of the above results except the failure of the passive layer, because there is no passive layer in the active system.

#### *2.1.1 Surface roughness*

The impact of particles makes the surface of the electrode rough and uneven. Normally, the rougher the surface, the higher the corrosion rate [14, 15]. When the electrode surface is hit by the particles, an impact cavity can form a crack or become sharp. Li and his colleagues [14, 15] used copper as a working electrode and researched the copper electrode. They found that it is easier for the electron to escape from the surface in the state of a sharp scratch than in the state of a gap. This shows that corrosion is easier in the pointed scratch mode than in the crevice and valley mode. For a completely rough surface, there is higher electron mobility and more freedom to react with the environment, so the surface is more electrochemically active. In addition, in the form of a sharp scratch, there is a tendency to lose more electrons and a more active surface. In contrast, the gap is nobler. This states that a galvanic couple can be formed, meaning that the pointed scratch acts as the anode, and the gaps act as the anode. Therefore, the localized cell formed can accelerate the corrosion process.

Burstein and his colleagues [16] investigated the relationship between surface roughness and pitting potential for 304 L stainless steel and found that surface condition is a critical parameter for determining pitting potential. They found that surface roughness caused by erosion has less pitting potential. He also noticed that during the erosion process, the potential of pitting decreases more than after the process. This shows that in addition to surface roughness, other parameters affect the pitting potential.

#### *2.1.2 Failure of the passive layer*

As mentioned, for a passive system, the surface of the material is covered by an oxide film that prevents the collision between the surface of the material and the environment. As a result of the erosion process, the surface of the passive layer may be particle collision or pitting will be damaged. Li et al. [17] and Zhao et al. [18] used the scratch test to simulate the impact of solid particles and obtained similar results to those obtained from the impact of solid particles. Some researchers [19, 20] reported that if the slurry concentration or velocity is high enough, it can replace the corrosion behavior; it means removing the passive area.

Failure of the passive film can increase pitting corrosion. When a particle hits the passive surface, its abrasion removes the passive surface and produces small holes. Burstein et al. [21] investigated the same effect on stainless steel and found that semistable pits can form after a short time below the rapid pitting potential. Also, it can increase the recurrence of semi-stable cavities compared with free erosion conditions. These small holes can shorten the period of pitting corrosion. The continued collision of particles ends the expansion of the small holes that created the large holes.

#### *2.1.3 Acceleration of mass transfer*

For a corrosion process, there are always two reactions: a cathodic reaction and an anodic reaction. When the electrolyte is fluid, it increases the transport of oxygen, the speed of the cathodic reaction increases, and as a result, the entire corrosion process improves. Also, the fluidity of solid particles in a solution leads to a disturbance in the fluid. Therefore, it can increase the transport processes of both reactants and corrosion products [12].

#### *2.1.4 Strain-hardening effect*

When particles collide with the material surface with a high and sufficient kinetic energy, they can cause both elastic and plastic deformations on the surface. This process includes a deforming effect, such as a misplaced lattice, deforming bands, or the protrusion of slip plates on the surface. Li and his colleagues [22] investigated the relationship between the effect of strain rate and electron work performance and found that a higher strain rate makes it easier for electrons to escape from the surface of the material and increases the tendency to corrosion. Li and his colleagues [23] also investigated the change of current density in stainless steel, caused by (changing) the strain rate in the solution %10*H*2*SO*4, and found that the current density increases with the increase of the strain rate. Mayozumi and his colleagues [24] found that cold working can affect the corrosion behavior of 304 stainless steel.

However, there are some conflicting results on the effect of strain and cold work. Lu et al. investigated the effect of strain on 304 stainless steel and found that in the quasi-elastic deformation process, neither elastic nor plastic deformation has significantly changed the corrosion rate of the electrode when it undergoes an anodic decomposition process. Mayozumi and his colleagues [24] also found that the effect of strain on corrosion is significantly dependent on the system. In Li's research [23], it is difficult to say that the failure of stainless steel passive film was due to tensile stress or strain rate that increased the current.

#### **2.2 The effect of corrosion on erosion**

Corrosion is a chemical process, specifically the breakdown of a surface layer of material. When solid particles hit the surface, the erosion process is much easier than when there is no corrosion flow at all, and this leads to a decrease in erosion resistance in the surface layer of the material [25, 26]. Processes such as roughening of the surface, preferential decomposition of the background phase, removal of the hardened surface, generation of vacancies, and chemical mechanical effect are studied in this section.

#### *2.2.1 Surface roughness*

When the microstructure of the material is not uniform, the decomposition process on the surface of these materials will not be uniform. Some areas are more susceptible to corrosion while others are not. As a conclusion, the more active areas act as the anode and the nobler areas act as the cathode. Therefore, we see metal decomposition in the anode. While this is not the case in the cathode. In these conditions, unevenness is created. Postlethwaite [27] investigated the erosive corrosion behavior of pipes in an aqueous slurry, with and without the presence of inhibitors. Inhibitors can slow down the corrosion process. He found that the rate of erosive corrosion and surface roughness can be reduced by adding inhibitors. Some researchers [28] explained that, because the erosion process is sensitive to the angle of impact of particles on the surface, corrosion by roughening the surface can change the angle of impact.

#### *2.2.2 Preferential decomposition of the matrix phase*

For some materials, the mechanical properties are enhanced by a secondary dispersed phase in the metal matrix composite (MMC). For example, the reinforced

#### *Erosion-corrosion DOI: http://dx.doi.org/10.5772/intechopen.109106*

phase can increase the surface hardness of the (MMC) and can make the material more resistant to wear and friction. The friction of the surfaces in contact is more resistant. For these materials, the mechanical properties are much more dependent on the reinforcing phases. When these materials are exposed to erosive corrosion, preferential decomposition always occurs at the junction of the ground and reinforcing phase, because it is more galvanically active; hence, they are more susceptible to corrosion [29]. This preferential degradation weakens the composite bonds and the metal substrate. Therefore, the reinforced composite may be destroyed earlier by the abrasive particles. This effect can reduce the mechanical properties of the surface of the material; as a result, the erosion resistance of the material decreases [30].