DE 3.2: Final report on towing tank experiments

Deliverable 3.2 "Final report on towing tank experiments" contains the results of the experimental activities performed in the CNR-INSEAN towing tank, devoted to the characterisation of the flow in correspondence of the test section of the Flame model, of the velocity field beneath the boundary layer, of wall pressure fluctuations and of the vibro-acoustic response of a reference metallic plate and of other three covered with different materials. In particular, the influence of the flow history in the test section, located approximately 2.5 m downstream the bow of the model has been analysed. Mean and fluctuating velocity quantities, scaling laws and models for the single point pressure spectra, cross spectral density and related semi-empirical models as well as wavenumber-frequency spectra are reported. Moreover, results relative to acceleration and acoustic pressure measurements, modal and data analyses are discussed. The main achievements and conclusions are summarised below. * The measured mean pressure difference acquired at the beginning and at the end of the wall pressure sensors array shown a very mild adverse pressure gradient most likely due to flow deceleration. * Velocity measurements indicated that the memory effects of the strong acceleration around the bow are confined to the outer layer. In fact, the mean velocity profile shown an higher values of the outer velocity with respect to the nominal carriage speed and a thicker boundary layer, if calculated in the position of the maximum stream-wise mean velocity. However, pressure gradient value is not sufficient to provide any shift from the log law as well as any difference in velocity statistics with respect to classical flat plate results. * Wall pressure spectra show some typical behaviours in the low frequency region that can be related to outer flow effects. The main findings of the present analysis are: i) a higher value of the scaled PSD amplitude in the low frequency range, below the logarithmic scale independent region, is observed, ii) the introduction of the dimensionless parameter D=d/d* seems to reduce the gap between ZPG and APG curves, iii) the stream-wise decay is faster and iv) the Goody model still gives a satisfactory representation of the single point pressure spectrum. Some confirmation of the above consideration can be found in literature, however the development of an analytical model able to take into account for pressure gradient effect needs some further effort and systematic comparisons between all the available data sets. * Semi-empirical models for the CSD can be used with suitable values of the free coefficients. In this regard, the sensitivity analysis performed on the values assumed by the coefficients of the Chase model, shown that they are significantly different from the original ones also in the case of ZPG equilibrium flow over a flat plate. Moreover, they depend on flow conditions even for very mild pressure gradient. However, if experimental data are not available, it is possible to define a reasonably small range of variation that can be useful to perform sensitivity analyses of the vibro-acoustic response. * The vibro-acoustic experimental campaign performed on a bare aluminium panel and on three metallic panels covered by different materials of same thickness had two main scopes: i) provide reference data for the development and validation of numerical and reduced order models and ii) analyse the different behaviour of material coatings usually adopted for this kind of applications.