Boundary Layer Wall Pressure Research Articles

The rôle of surface shear stress fluctuations in the generation of boundary layer noise

The influence of unsteady wall shear stress on boundary layer noise and wall pressure fluctuations is discussed. It is argued that in the acoustic analogy theory of boundary layer noise the surface shear stress “dipole” characterizes acoustic propagation and not generation. Analytical results are presented in support of this view which, in addition, indicate that the effect of the surface dipole is to dininish rather than enhance boundary layer radiation at low Mach numbers.

Optimal discrimination against a convecting and exponentially decaying noise field and against all‐pole stochastic processes

A closed‐form solution is presented for optimal discrimination against a spatially homogeneous, convecting, and exponentially decaying random field. The optimal procedure requires modification to the conventional processing only for the two end sensors of a line array of equispaced sensors. The same feature, i.e., confinement of the modification to sensors at the two ends of the array, applies for a class of noise fields. These noise fields are discrete in space, defined only at equispaced locations in space and are modeled by what is called purely autoregressive or all‐pole processes. Relevance of the above work for measurement of low wave‐number and acoustic contents of a low‐speed turbulent boundary layer wall pressure field is discussed.

Measurements of the low wave number components of turbulent boundary layer wall pressure fluctuations with zero and adverse pressure gradients

Two rectangular plates with approximately clamped boundary conditions were used as spatial filters to measure the low wave number components of the wall pressure fluctuations beneath a turbulent boundary layer. The plates were designed to provide low wave number measurements at higher frequencies and wave numbers than previous plate experiments in order to provide data comparable to measurements made using microphone arrays. Measurements were made in the lower test section wall of the MIT low turbulence subsonic wind tunnel under zero pressure gradient and adverse pressure gradient conditions. The zero pressure gradient data lies in the wave number frequency range 2.5 <ωδ*/U∞<10 and 0.5 <k1δ*<1.8 while the adverse gradient data lies in the range 4.5 <ωδ*/U∞<23 and 0.9 <k1δ*<4.6. Momentum thickness Reynolds numbers ranged from 6000 to 11 000 for the zero pressure gradient case and 11 000 to 16 000 for the adverse pressure gradient case. When normalized on outer variables the adverse gradient data do not exhibit increased levels from those of the zero pressure gradient data. [Work supported by the Sonar Technology Program, Office of Naval Research.]

Measured spatial filtering characteristics of membranes and plates

Two rectangular plates and a membrane were designed to be used as spatial filters for measuring the low‐wavenumber components of turbulent boundary layer wall pressure fluctuations. For such stuctures it is convenient to define wavenumber filter‐shape functions which are obtained from spatial transforms of the normal mode shapes. This suggests that it is possible to determine experimentally the wavenumber filter shapes by measuring the normal mode shapes. Detailed measurements of selected mode shapes were made for a membrane, a clamped plate, and a simply supported plate. The mode‐shape data was then transformed numerically to obtain typical wavenumber filter shapes for the structures. These measured wavenumber filter shapes compare favorably with theoretical predictions; however, to obtain reliable results it was necessary to attain both frequency and spatial matching of the excitation to the desired resonant mode. [Work supported by the Sensor Technology Program, ONR.]

Excitation of a Helmholtz resonator under a turbulent boundary layer

A zero-pressure-gradient boundary layer on the fuselage of a glider was used to excite a Helmholtz resonator. The resonator was constructed so the orifice was flush with the surface. Nominal resonator frequencies were chosen so they would tune with different portions of the boundary layer wall pressure spectrum. The resonators were excited at both the Helmholtz frequency and a standing wave frequency. The results show a shift in the Helmholtz frequency when the boundary layer is present. This shift indicates the degree to which the turbulence interacts with the acoustic motion in the orifice and modifies the end correction. In several cases extremely high cavity responses were observed. These cases produced measurable sound in the free stream. The existence of this phenomenon correlates with the parameter ωd/u* (ω = resonance frequency, d = orifice diameter, u* = friction velocity). [This work was supported by NASA Ames Research Center.]

Optimizing loss-compensated long delay lines : Coldren, L. A. 20 (January 1973) 17

The drag reduction characteristics of certain high molecular weight polymers have been studied by various investigators. Because of the polymer’s ability to reduce turbulent shear stress and dependence of the boundary layer wall pressure spectral amplitude on the shear stress, polymer has the potential to suppress noise and vibration caused by the boundary layer unsteady pressures. Compared to its effect on drag reduction, polymer additive effects on turbulent boundary layer (TBL) wall pressure fluctuations have received little attention. Kadykov and Lyamshev [Sov. Phys. Acoust. 16 (1970) 59], Greshilor et al. [Sov. Phys. Acoust. 21 (1975) 247] showed that drag reducing polymer additives do indeed reduce wall pressure fluctuations, but they have not established any scaling relationship which effectively collapse data. Some effort has been made by Timothy et al. [JASA 108 (1) (2000) 71] at Penn State University to develop a scaling relationship for TBL wall pressure fluctuations that are modified by adding drag reducing polymer to pure water flow. This paper presents a theoretical model based on the work of the Timothy et al. team at ARL, Penn State University. Through this model one can estimate, reduction in TBL flow induced noise and vibration for rigid smooth surfaces due to release of drag reducing polymers in boundary layer region. Using this theoretical model, flow noise as experienced by a typical flush mounted hydrophone has been estimated for a smooth wall plate as a function of polymer additive concentration. Effect of non-dimensionalisation of the wall pressure fluctuations frequency spectra with traditional outer, inner and mixed flow variables will also be addressed in the paper. The paper also covers a model based on molecular relaxation time in polymer additives which not only reduce drag but also flow induced noise up to certain polymer concentration.

Ferroelectric non-linearities in transudcer ceramics : Woollet, R. S. LeBlanc, C. L. 20 (January 1973) 24

The drag reduction characteristics of certain high molecular weight polymers have been studied by various investigators. Because of the polymer’s ability to reduce turbulent shear stress and dependence of the boundary layer wall pressure spectral amplitude on the shear stress, polymer has the potential to suppress noise and vibration caused by the boundary layer unsteady pressures. Compared to its effect on drag reduction, polymer additive effects on turbulent boundary layer (TBL) wall pressure fluctuations have received little attention. Kadykov and Lyamshev [Sov. Phys. Acoust. 16 (1970) 59], Greshilor et al. [Sov. Phys. Acoust. 21 (1975) 247] showed that drag reducing polymer additives do indeed reduce wall pressure fluctuations, but they have not established any scaling relationship which effectively collapse data. Some effort has been made by Timothy et al. [JASA 108 (1) (2000) 71] at Penn State University to develop a scaling relationship for TBL wall pressure fluctuations that are modified by adding drag reducing polymer to pure water flow. This paper presents a theoretical model based on the work of the Timothy et al. team at ARL, Penn State University. Through this model one can estimate, reduction in TBL flow induced noise and vibration for rigid smooth surfaces due to release of drag reducing polymers in boundary layer region. Using this theoretical model, flow noise as experienced by a typical flush mounted hydrophone has been estimated for a smooth wall plate as a function of polymer additive concentration. Effect of non-dimensionalisation of the wall pressure fluctuations frequency spectra with traditional outer, inner and mixed flow variables will also be addressed in the paper. The paper also covers a model based on molecular relaxation time in polymer additives which not only reduce drag but also flow induced noise up to certain polymer concentration.

Use of correlation technique for estimating in-flight noise radiated by wing-mounted jet engines on a fuselage

Turbulent boundary layer pressure fluctuations and noise radiated by jet engines form two major sources of pressure fluctuations on the exterior of many commercial jet fuselages. The expressions for correlations and mean square pressures of two statistically independent noise sources are derived. A method of decomposing the two pressure fields is illustrated using flight test measurements. In flight, the jet engine noise contribution is separated from the turbulent boundary layer wall pressure fluctuations at Mach 0·78, aft fuselage location, and Mach 0·60 forward fueslage location. The flight test measurements indicated that the turbulent boundary layer pressure fluctuations and the noise radiated by jet engines are of the same order of magnitude at Mach 0·78, 25 000 ft (7620 m) altitude at the aft fuselage location.