Provide function to convert from Te to Tp using ITTC approximation (#210)

* Provide function to convert from Te to Tp using ITTC approximation

* Apply suggestions from code review

Co-authored-by: Adam Keester <[email protected]>

Author: mbruggs

mhkit/tests/wave/test_resource_metrics.py

@@ -201,6 +201,26 @@ def test_moments(self):
                     self.assertLess(error, 0.01)
 
 
+    def test_energy_period_to_peak_period(self):
+        # This test checks that if we perform the
+        # Te to Tp conversion, we create a spectrum
+        # (using Tp) that has the provided Te.
+        Hs = 2.5
+        Te = np.linspace(5, 20, 10)
+        gamma = np.linspace(1, 7, 7)
+
+        for g in gamma:
+            for T in Te:
+                Tp = wave.resource.energy_period_to_peak_period(T, g)
+
+                f = np.linspace(1 / (10 * Tp), 3 / Tp, 100)
+                S = wave.resource.jonswap_spectrum(f, Tp, Hs, g)
+
+                Te_calc = wave.resource.energy_period(S).values[0][0]
+
+                error = np.abs(T - Te_calc)/Te_calc
+                self.assertLess(error, 0.01)
+
 
     def test_metrics(self):
        for file_i in self.valdata2.keys(): # for each file MC, AH, CDiP
@@ -356,6 +376,6 @@ def test_plot_monthly_cumulative_distribution(self):
 
         self.assertTrue(isfile(filename))
 
-        
+
 if __name__ == '__main__':
     unittest.main()

mhkit/wave/resource.py

@@ -591,6 +591,34 @@ def energy_flux(S, h, deep=False, rho=1025, g=9.80665, ratio=2):
     return J
 
 
+def energy_period_to_peak_period(Te, gamma):
+    """
+    Convert from spectral energy period (Te) to peak period (Tp) using ITTC approximation for JONSWAP Spectrum.
+
+    Approximation is given in "The Specialist Committee on Waves, Final Report
+    and Recommendations to the 23rd ITTC", Proceedings of the 23rd ITTC - Volume
+    2, Table A4.
+
+    Parameters:
+    ----------
+    Te: float or array
+        Spectral energy period [s]
+    gamma: float or int
+        Peak enhancement factor for JONSWAP spectrum
+
+    Returns
+    -------
+    Tp: float or array
+        Spectral peak period [s]
+    """
+    assert isinstance(Te, (float, np.ndarray)), 'Te must be a float or a ndarray'
+    assert isinstance(gamma, (float, int)), 'gamma must be of type float or int'
+
+    factor = 0.8255 + 0.03852*gamma - 0.005537*gamma**2 + 0.0003154*gamma**3
+
+    return Te / factor
+
+
 def wave_celerity(k, h, g=9.80665, depth_check=False, ratio=2):
     """
     Calculates wave celerity (group velocity)