Simultaneous Surface Pressure Measurements Research Articles

Background-oriented schlieren imaging of flow around a circular cylinder at low Mach numbers

The background-oriented schlieren (BOS) imaging method has, for the first time, been applied in the investigation of the flow around a circular cylinder at low Mach numbers ( $$M<0.1$$ ). The measurements were conducted in the high pressure wind tunnel, Gottingen, with static pressures ranging from 0.1 MPa to 6.0 MPa and covered a range of the Reynolds numbers of $$0.1\times 10^6 \le Re \le 6.0\times 10^6$$ . Even at ambient pressure and the lowest Reynolds number investigated, density gradients associated with the flow around the cylinder were recorded. The signal-to-noise ratio of the evaluated gradient field improved with increasing stagnation pressure. The separation point could easily be identified with this non-intrusive measurement technique and corresponds well to simultaneous surface pressure measurements. The resulting displacement field is in principle of qualitative nature as the observation angle was parallel to the cylinder axis only in a single point of the recorded images. However, it has been possible to integrate the density field along the surface of the cylinder by successive imaging at incremental angular positions around the cylinder. This density distribution has been found to agree well with the pressure measurements and with potential theory where appropriate.

Wind Tunnel Test of Symmetrical Twin-Tower Tall Building Model

In this paper, taking symmetrical twin-tower tall building model as an example, a wind tunnel test with simultaneous surface pressure measurement, in 7 wind directions, was carried out. Integration of the surface pressures leads to base moment coefficient, of which the amplitude and frequency-domain characteristics were analyzed and compared with those of single tall building model. The result shows that mean value and root mean square of the interfering tower, in all wind directions, are basically the same as those of single tower, while in 0o wind direction, they differ greatly; mean value and root mean square of the interfered tower differ significantly, in every wind directions, from those of single tower. In 0o wind direction, the reduced spectrum of along-wind and across-wind base moment coefficient is greatly different from that of the single tower; in 90o wind direction, the along-wind base moment coefficient reduced spectrum for interfered tower is different from Davenport spectrum, while the peak value of across-wind base moment coefficient is half the corresponding value of single tower.

PIV-based pressure fluctuations in the turbulent boundary layer

The unsteady pressure field is obtained from time-resolved tomographic particle image velocimetry (Tomo-PIV) measurement within a fully developed turbulent boundary layer at free stream velocity of U ∞ = 9.3 m/s and Reθ = 2,400. The pressure field is evaluated from the velocity fields measured by Tomo-PIV at 10 kHz invoking the momentum equation for unsteady incompressible flows. The spatial integration of the pressure gradient is conducted by solving the Poisson pressure equation with fixed boundary conditions at the outer edge of the boundary layer. The PIV-based evaluation of the pressure field is validated against simultaneous surface pressure measurement using calibrated condenser microphones mounted behind a pinhole orifice. The comparison shows agreement between the two pressure signals obtained from the Tomo-PIV and the microphones with a cross-correlation coefficient of 0.6 while their power spectral densities (PSD) overlap up to 3 kHz. The impact of several parameters governing the pressure evaluation from the PIV data is evaluated. The use of the Tomo-PIV system with the application of three-dimensional momentum equation shows higher accuracy compared to the planar version of the technique. The results show that the evaluation of the wall pressure can be conducted using a domain as small as half the boundary layer thickness (0.5δ99) in both the streamwise and the wall normal directions. The combination of a correlation sliding-average technique, the Lagrangian approach to the evaluation of the material derivative and the planar integration of the Poisson pressure equation results in the best agreement with the pressure measurement of the surface microphones.

Flow visualization of conical vortices on flat roofs with simultaneous surface pressure measurement

Wind tunnel and full-scale pressure studies of flow over low-rise buildings have repeatedly shown that on the roof, the largest mean and peak suction values are observed for taps beneath the conical “delta-wing type” corner vortices that occur for oblique winds. To better understand the flow mechanism which produces these negative pressure coefficients, a flow visualization study of conical vortex behaviour was performed in the wind tunnels of Colorado State University (CSU) and at full scale at Texas Tech University (TTU). The mean position and size of the vortices as a function of wind direction is presented. Pressures were also simultaneously measured beneath the vortex visualization plane in the wind tunnel for the worst case wind directions. These pressure profiles were correlated with the digitally enhanced images of the vortex flow. The greatest suction was found to follow directly beneath the moving vortex core. For smooth flow, the magnitude of the suction beneath the core was seen to vary inversely with the vortex size, but no relationship between vortex size and suction could be seen for turbulent flow.