Stability of momentumless wakes

The caudal fin of swimming animals can be modelled as a thrust-producing flapping foil. When considered alone, such a foil produces on average a jet wake with a positive net momentum. It has been argued that the instability characteristics of these averaged wakes are linked to the propulsion efficiency of swimming animals. Here, we reconsider this question by taking into account both the thrust and the drag exerted on a self-propelled swimming body. To do so, we study the stability of a family of momentumless wakes, constructed as the Oseen approximation of a force doublet moving at constant velocity. By performing a local stability analysis, we first show that these wakes undergo a transition from absolute to convective instability. Then, using the time stepper approach by integrating the linearised Navier–Stokes system, we investigate the global stability and reveal the influence of a non-parallel base flow as well as the role of the locally absolutely unstable upstream region in the wake. Finally, to complete the global scenario, we address the nonlinear evolution of the wake disturbance. These results are then discussed in the context of aquatic locomotion. According to the present stability results, and assuming the Oseen approximation whose validity has been assessed only for moderate Reynolds number, the momentumless wake of aquatic animals is generally stable, whereas the corresponding thrust part is unstable. It is therefore essential to consider all forces exerted on a self-propelled animal when discussing its wake stability and its propulsion efficiency.

Experimental investigation on the momentumless wake using trailing edge blowing

A developed plate stator model with and without trailing edge blowing (TEB) is studied using experimental methods. Wake characteristics of flow over the stator in the three-dimensional wake regimes are studied using hot-wire anemometry (HWA) and particle image velocimetry (PIV) techniques. First, the mean velocity profiles have been measured in the wake of the stator using HWA. Four wake characteristics have been obtained through momentum thickness judgments: pure wake, weak wake, momentumless wake, and jet. These velocity profiles show some differences in momentum deficit for the four cases. Then, the velocity spectra of the pure wake and momentumless wake obtained through the HWA measurements showed that TEB can eliminate the shedding vortex of the stator. Characteristic length scales based on the wake turbulent intensity profiles showed that the momentumless wake can reduce the wake width and depth. PIV measurement is carried out to measure the flow field of the four wakes. Finally, the application of TEB approaching momentumless wake status is used on an industrial ventilation low-pressure axial fan to assess noise reduction. The results show that TEB can make the outlet of the stator uniform, reduce velocity fluctuation, destroy the vorticity structure downstream of the stator, and reduce interaction noise level of the stator and rotor.

Experimental and Numerical Investigation on Aeroacoustic Sound of Stator-Rotor System With Trailing Edge Blowing

The noise radiation from industrial ventilation fans becomes one of the key points during design process, and reducing the noise level is the effort of all investigations. Recently, the trailing edge blowing (TEB) technique is used in the field of aeroengines and multistage turbo machines, which is used to reduce the interaction between wake and blade rows to improve the aeroacoustic performance of the rotating machines. In this paper the TEB technique is employed in the low speed ventilation fan with upstream stator. How to reduce blade passing frequency (BPF) noise and total noise level was investigated with experimental measurements and noise prediction based on CFD simulation flow filed. Based on the experimental measurements and noise predictions, the inlet flow of the low speed rotor is the main noise source for noise radiation. With TEB wake management and TEB up to the momentumless wake, the inlet flow for rotor was more uniform and shed wake from the rotor blades was weaken. The noise level from the fans was reduced.