Energy conservation in the 3D Euler equations on $\mathbb{T}^2\times \mathbb{R}_+$
The aim of this paper is to prove energy conservation for the incompressible Euler equations in a domain with boundary. We work in the domain $\mathbb{T}^2\times\mathbb{R}_+$, where the boundary is both flat and has finite measure. However, first we study the equations on domains without boundary (the whole space $\mathbb{R}^3$, the torus $\mathbb{T}^3$, and the hybrid space $\mathbb{T}^2\times\mathbb{R}$). We make use of some of the arguments of Duchon \& Robert ({\it Nonlinearity} {\bf 13} (2000) 249--255) to prove energy conservation under the assumption that $u\in L^3(0,T;L^3(\mathbb{R}^3))$ and one of the two integral conditions \begin{equation*} \lim_{|y|\to 0}\frac{1}{|y|}\int^T_0\int_{\mathbb{R}^3} |u(x+y)-u(x)|^3\,d x\,d t=0 \end{equation*} or \begin{equation*} \int_0^T\int_{\mathbb{R}^3}\int_{\mathbb{R}^3}\frac{|u(x)-u(y)|^3}{|x-y|^{4+\delta}}\,d x\,d y<\infty,\qquad\delta>0, \end{equation*} the second of which is equivalent to requiring $u\in L^3(0,T;W^{\alpha,3}(\mathbb{R}^3))$ for some $\alpha>1/3$. We then use the first of these two conditions to prove energy conservation for a weak solution $u$ on $D_+:=\mathbb{T}^2\times \mathbb{R}_+$: we extend $u$ a solution defined on the whole of $\mathbb{T}^2\times\mathbb{R}$ and then use the condition on this domain to prove energy conservation for a weak solution $u\in L^3(0,T;L^3(D_+))$ that satisfies \begin{equation*} \lim_{|y|\to 0} \frac{1}{|y|}\int^{T}_{0}\iint_{\mathbb{T}^2}\int^\infty_{|y|}|u(t,x+y)-u(t,x)|^3 \,d x_3 \,d x_1 \,d x_2 \,d t=0, \end{equation*} and certain continuity conditions near the boundary $\partial D_+=\{x_3=0\}$.