Jonathan F. Morrison

The generative mechanism of the pressure field is fundamental in turbulence research and flow control, especially when subject to turbulent separation at high Reynolds numbers. To this end, we propose a physics-based framework—momentum decomposition of the pressure field—which uses the momentum equations to decompose the spatial pressure gradient into quantifiable momentum budgets. Each momentum budget corresponds to a distinct physical mechanism, enabling a rigorous decomposition and quantitative evaluation of the inherent mechanisms governing pressure generation. Incorporating results of a direct numerical simulation of laminar cylinder flow, this method is first demonstrated, after which the three-dimensional large eddy simulation results of a complex turbulent bluff body wake at Re D = 1.88 × 10 5 are analyzed with a comparison of flow control. In the near wake, the bulk of low pressure originates from the streamwise convection of the centripetal flow outside the wake, which manifests as a streamline curvature effect from the term u ¯ ∂ v ¯ / ∂ x , while the pressure variation inside the wake receives main contributions from Reynolds stresses. Under flow control, due to the streamline curvature modification near the base edge, the pulsed jet produces favorable pressure gradients from the mean flow term u ¯ ∂ v ¯ / ∂ x contributing to the bulk of base-pressure recovery. Overall, the proposed framework provides a fundamental flow diagnostic tool and a rigorous foundation for designing and analyzing flow control strategies, since pressure control can be rationally linked to the momentum budgets manipulation, generally applicable to flow regimes governed by the Navier-Stokes equations.

The effect of wall deformations on wall turbulence is a topic of fundamental importance for the advancement of flow control strategies aimed at reducing the frictional drag. Dynamic wall deformations in the form of wall-normal of in-plane oscillations represents an area that is yet to be fully explored, and that can open a new realm of drag control strategies, as well as provide fresh insights into the structure of wall turbulence. In this study, we present several results from direct numerical simulations aimed at understanding the response of wall turbulence to standing wave-like wall motion, arranged in a checkerboard pattern. More specifically, here we target a low Reynolds number turbulent channel flow, featuring standing wave-like dynamic wall deformations on both bottom and top walls, with parameterizations in terms of the frequency of oscillations, roughness height, and spacing. We quantify the effect that this type of wall motion can have on the skin friction drag, various turbulent statistics, and turbulent flow structures. In addition, taking advantage of the periodicity and the symmetry of the flow, an improved phase-averaging procedure is introduced, which enhances the smoothness of the data. The results show that this type of dynamic wall deflection can have a significant effect on the turbulent flow in proximity to the wall, and that the variation of the spatial wavenumbers of the wall deflections can make a big difference. A slight total drag reduction, in the order of 2%, was observed for some combinations of wavenumbers and frequencies, especially for the highest streamwise wavenumber case.

A compressible large eddy simulation (LES) is performed to study a pulsed jet actuator that is used to control a turbulent axisymmetric bluff body wake. The actuator is driven at low-frequency (). The numerical scheme and a suitable boundary condition for the pulsed jet are validated, showing good agreement with experimental results. A comparison of the velocity boundary condition and the moving boundary condition shows that, in the vicinity of the orifice/slot and in the downstream region, the results from these two methods are identical, while the fluid behaviour inside the cavity shows difference. An analysis of the pulsed jet actuator shows that the phase lag of the cavity pressure is determined by the integration of the diaphragm motion and the pulsed jet. The mean total pressure distribution shows that the total pressure loss is concentrated in the vicinity of the slot. Dynamic mode decomposition (DMD) on the pressure field is used to extract coherent structures which oscillate with the same frequency as that of the diaphragm motion. Some small-scale high-frequency structures are also apparent.

Among the different passive and active techniques for skin friction drag reduction for turbulent boundary layers, near wall forcing through moving walls is one of the most promising techniques at low Reτ. Fewer studies have looked at the mechanism at high Reτ, closer to flight conditions, largerly because, in this regime, numerical simulations become harder and experiments more challenging. To that end, there is the need of a systematic study for different surface waves and flow conditions. A new model using a kagome lattice is presented. It will be tested in the 10x5 wind tunnel at Reτ ≈ 8000. This work introduces an experimental setup which combines simultaneous measurements of surface displacement and velocity in the boundary layer. Here the results from a shorted version of the model at Reτ ≈ 1000 are presented to demonstrate the capability of the experimental setup.

A three-layer asymptotic structure for turbulent pipe flow is proposed revealing, in terms of intermediate variables, the existence of a Reynolds-number-invariant logarithmic region for the streamwise mean velocity and variance. The formulation proposes a local velocity scale (which is not the friction velocity) for the intermediate layer and results in two overlap layers. We find that the near-wall overlap layer is governed by a power law for the pipe for all Reynolds numbers, whereas the log law emerges in the second overlap layer (the inertial sublayer) for sufficiently high Reynolds numbers () and therefore the approach to an asymptotically invariant state can be expected to be slow.