# On URANS Congruity with Time Averaging: Analytical laws suggest improved models

## TL;DR

This paper demonstrates that a specific kinematic specification of turbulence lengthscale in a 1-equation URANS model satisfies key physical and mathematical laws without additional modifications, improving model consistency.

## Contribution

It introduces a kinematic lengthscale formulation that inherently satisfies five fundamental laws for URANS models, enhancing their theoretical foundation.

## Key findings

- The proposed lengthscale ensures compliance with time window and wall behavior laws.
- The model maintains bounded energy over time.
- It achieves statistical equilibrium without model adjustments.

## Abstract

The standard $1-$equation model \ of turbulence was first derived by Prandtl and has evolved to be a common method for practical flow simulations. Five fundamental laws that any URANS model should satisfy are \[ \begin{array} [c]{ccc} \textbf{1.} & \text{Time window:} & \begin{array} [c]{c} \tau\downarrow 0\text{ implies }v_{\text{{\small URANS}}}\rightarrow u_{\text{{\small NSE}}}\text{ \&}\\ \text{ }\tau\uparrow\text{implies }\nu_{T}\uparrow \end{array} \\ \textbf{2.} & l(x)=0\ \text{at walls:} & l(x)\rightarrow 0\text{ as }x\rightarrow walls,\\ \textbf{3.} & \text{ Bounded energy:} & \sup_{t}\int\frac{1} {2}|v(x,t)|^{2}+k(x,t)dx<\infty\\ \textbf{4.} & \begin{array} [c]{c} \text{Statistical }\\ \text{equilibrium:} \end{array} & \lim\sup_{T\rightarrow\infty}\frac{1}{T}\int_{0}^{T}\varepsilon _{\text{model}}(t)dt=\mathcal{O}\left( \frac{U^{3}}{L}\right) \\ \textbf{5.} & \begin{array} [c]{c} \text{Backscatter}\\ \text{possible:} \end{array} & \text{(without negative viscosities)} \end{array} \] This report proves that a \textit{kinematic} specification of the model's turbulence lengthscale by \[ l(x,t)=\sqrt{2}k^{1/2}(x,t)\tau\text{ }, \] where $\tau$\ is the time filter window, results in a $1-$equation model satisfying Conditions 1,2,3,4 without model tweaks, adjustments or wall damping multipliers.