Sound source mechanisms in under-expanded impinging jets

Effects of Nozzle Pressure Ratio and Nozzle-to-Plate Distance to Flowfield Characteristics of an Under-Expanded Jet Impinging on a Flat Surface

“…As discussed in many previous studies, for examples those of Henderson [9], Sinibaldi et al [18], Snedeker et al [41], Donaldson et al [42], Alvi and Iyer [43], and Wilke and Sesterhenn [7], the configuration of a turbulent jet impinging on a surface has a complex flow field, which can be roughly described as three regions. These include an upstream region, where the flow has behaviors like a free jet; an impingement region, where the flow structure is affected by the presence of the impingement plane; and a wall jet region, where the flow develops in the radial direction along the impingement surface.…”

“…The seeding particles can be considered to follow the flow properly if the particle relaxation time τ p is much smaller than the flow timescale τ f , i.e., Stokes number Stk 1 (Erdem et al [31], Ragni et al [32]). Providing the properties of the DEHS particles, air properties, and jet velocity for each experimental measurement in the current study, one can obtain the particle response time is τ p = 2.82 × 10 −6 s and τ f = 3.81 × 10 −4 s. This yields the Stokes number Stk 0.031, thus ensuring reliable fluid flow detection based on the seeding particles and laser illumination (Sinibaldi et al [18]). Table 1 shows typical flow conditions calculated at the nozzle exit for the free jets and impinging jets at various nozzle-to-plate gaps and various values of NPRs.…”

“…For velocity measurements of high-speed flows using laser-based techniques, Scarano [27], Alvi et al [28], and Mitchell et al [29] discussed the importance of seeding particles because particle lag could occur in flow regions of very high velocity gradients, such as shock waves and wall jets. Following the discussions in Raffel et al [26] and in the study of Sinibaldi et al [18], particle response to a constant flow acceleration is defined by…”

“…The Stokes number (Stk) is defined as the ratio between the particle relaxation time τ p and the characteristics flow timescale τ f . Samimy and Lele [30] suggested an estimation of the flow timescale by assuming the particle maximum slip velocity as V j (Sinibaldi et al [18]), as follow…”

“…The feedback loop initiates as instability waves in the jet shear layer, then grow into large-scale vortices as they travel downstream (Brown and Roshko [13], Tam and Ahuja [14], Henderson and Powell [15]). The impact of the large-scale vortices on the solid surface produces pressure fluctuations and acoustic waves propagating upstream (Krothapalli et al [3], Henderson [16], Henderson et al [17], Sinibaldi et al [18], Gojon et al [19]). These upstream propagated waves reach the nozzle outlet and generate the instability waves, closing the feedback loop (Henderson and Powell [15], Ho and Nosseir [20], Nosseir and Ho [21]).…”