diff --git a/Projects/HX_PINN/HX_model.py b/Projects/HX_PINN/HX_model.py
new file mode 100644
index 00000000..6f083f86
--- /dev/null
+++ b/Projects/HX_PINN/HX_model.py
@@ -0,0 +1,235 @@
+# PINN for a tube-in-tube heat exchanger (steady 2D xr)
+# - Uses automatic differentiation for PDE residuals
+# - Prescribes velocity fields u_h(x,r), u_c(x,r)
+# - Solves for Th, Tw, Tc
+
+import torch
+import torch.nn as nn
+
+# ====== User-configurable physical constants (nondimensional if you scaled) ======
+Pe_h = 100.0  # Peclet (hot side)  = u_h*L/alpha_h
+Pe_c = 120.0  # Peclet (cold side) = u_c*L/alpha_c
+alpha_w_ratio = 1.0  # wall alpha relative to hot-side alpha if nondim
+Ri = 1.0  # inner radius (nondim, after scaling)
+Ro = 1.3  # outer radius
+
+device = "cuda" if torch.cuda.is_available() else "cpu"
+dtype = torch.float32  # float32 is usually fine; use float64 if residuals are stiff
+
+
+# ====== Velocity profiles (nondim) ======
+def u_h(x, r):
+    # Poiseuille in a circular tube: u_max*(1 - (r/Ri)^2). Here Ri=1 in nondim.
+    return 2.0 * (1.0 - r**2)  # average=1 if scaled; adjust factor to match Pe_h
+
+
+def u_c(x, r):
+    # Simple plug profile in annulus as a first approximation
+    return torch.ones_like(r)
+
+
+# ====== Network ======
+class MLP(nn.Module):
+    def __init__(self, in_dim=2, hidden=128, depth=7, out_dim=3, act=nn.SiLU):
+        super().__init__()
+        layers = [nn.Linear(in_dim, hidden), act()]
+        for _ in range(depth - 1):
+            layers += [nn.Linear(hidden, hidden), act()]
+        # Separate heads for Th, Tw, Tc improves conditioning
+        self.backbone = nn.Sequential(*layers)
+        self.head_h = nn.Linear(hidden, 1)
+        self.head_w = nn.Linear(hidden, 1)
+        self.head_c = nn.Linear(hidden, 1)
+
+        # Xavier init helps with tanh/SiLU nets
+        def init(m):
+            if isinstance(m, nn.Linear):
+                nn.init.xavier_uniform_(m.weight)
+                nn.init.zeros_(m.bias)
+
+        self.apply(init)
+
+    def forward(self, x, r):
+        z = torch.stack([x, r], dim=-1)
+        f = self.backbone(z)
+        Th = self.head_h(f)
+        Tw = self.head_w(f)
+        Tc = self.head_c(f)
+        return Th, Tw, Tc
+
+
+net = MLP().to(device).to(dtype)
+
+
+# ====== Utility: gradients via autograd ======
+def grads(y, x):
+    return torch.autograd.grad(
+        y, x, grad_outputs=torch.ones_like(y), create_graph=True
+    )[0]
+
+
+# ====== Collocation samplers ======
+def sample_in_hot(N):
+    # x in [0,1], r in [0,Ri]
+    x = torch.rand(N, 1, device=device, dtype=dtype)
+    r = Ri * torch.sqrt(torch.rand(N, 1, device=device, dtype=dtype))  # area-uniform
+    return x.requires_grad_(True), r.requires_grad_(True)
+
+
+def sample_in_wall(N):
+    x = torch.rand(N, 1, device=device, dtype=dtype)
+    r = torch.sqrt(
+        (Ro**2 - Ri**2) * torch.rand(N, 1, device=device, dtype=dtype) + Ri**2
+    )
+    return x.requires_grad_(True), r.requires_grad_(True)
+
+
+def sample_in_cold(N):
+    x = torch.rand(N, 1, device=device, dtype=dtype)
+    r = (Ro + (Ro)) * 0.5 * 0 + (
+        torch.rand(N, 1, device=device, dtype=dtype) * (Ro - Ri) + Ri
+    )  # simple annulus
+    return x.requires_grad_(True), r.requires_grad_(True)
+
+
+def sample_interface_Ri(N):
+    x = torch.rand(N, 1, device=device, dtype=dtype)
+    r = torch.full_like(x, Ri, requires_grad=True)
+    return x, r
+
+
+def sample_interface_Ro(N):
+    x = torch.rand(N, 1, device=device, dtype=dtype)
+    r = torch.full_like(x, Ro, requires_grad=True)
+    return x, r
+
+
+def sample_inlet_hot(N):
+    x = torch.zeros(N, 1, device=device, dtype=dtype, requires_grad=True)
+    r = Ri * torch.sqrt(torch.rand(N, 1, device=device, dtype=dtype))
+    return x, r
+
+
+def sample_inlet_cold_counterflow(N):
+    x = torch.ones(N, 1, device=device, dtype=dtype, requires_grad=True)
+    r = torch.rand(N, 1, device=device, dtype=dtype) * (Ro - Ri) + Ri
+    return x, r
+
+
+def sample_outlet_hot(N):
+    x = torch.ones(N, 1, device=device, dtype=dtype, requires_grad=True)
+    r = Ri * torch.sqrt(torch.rand(N, 1, device=device, dtype=dtype))
+    return x, r
+
+
+def sample_outlet_cold(N):
+    x = torch.zeros(N, 1, device=device, dtype=dtype, requires_grad=True)
+    r = torch.rand(N, 1, device=device, dtype=dtype) * (Ro - Ri) + Ri
+    return x, r
+
+
+# ====== Boundary condition targets (nondim) ======
+T_h_in = 1.0  # scale so hot inlet is 1
+T_c_in = 0.0  # cold inlet is 0
+
+# ====== Training loop ======
+opt = torch.optim.Adam(net.parameters(), lr=1e-3)
+
+for it in range(20000):
+    opt.zero_grad()
+
+    # ----- Interior residuals -----
+    Nh = Nw = Nc = 512
+    xh, rh = sample_in_hot(Nh)
+    xw, rw = sample_in_wall(Nw)
+    xc, rc = sample_in_cold(Nc)
+
+    Th, Tw, Tc = net(xh, rh)
+    Th_x = grads(Th, xh)
+    Th_r = grads(Th, rh)
+    Th_xx = grads(Th_x, xh)
+    Th_rr = grads(Th_r, rh)
+    # Hot PDE: u_h dTh/dx = (1/Pe_h)*(Th_rr + (1/r) Th_r + Th_xx)  (choose to keep xx or drop)
+    uh = u_h(xh, rh)
+    hot_res = uh * Th_x - (1.0 / Pe_h) * (Th_rr + (1.0 / rh) * Th_r + Th_xx)
+
+    (Tw_,) = net(xw, rw)[1:2]  # only Tw
+    Tw_x = grads(Tw_, xw)
+    Tw_r = grads(Tw_, rw)
+    Tw_xx = grads(Tw_x, xw)
+    Tw_rr = grads(Tw_r, rw)
+    wall_res = Tw_rr + (1.0 / rw) * Tw_r + Tw_xx  # alpha_w absorbed in scaling
+
+    Tc = net(xc, rc)[2]
+    Tc_x = grads(Tc, xc)
+    Tc_r = grads(Tc, rc)
+    Tc_xx = grads(Tc_x, xc)
+    Tc_rr = grads(Tc_r, rc)
+    uc = u_c(xc, rc)
+    cold_res = uc * Tc_x - (1.0 / Pe_c) * (Tc_rr + (1.0 / rc) * Tc_r + Tc_xx)
+
+    L_pde = hot_res.pow(2).mean() + wall_res.pow(2).mean() + cold_res.pow(2).mean()
+
+    # ----- Interface continuity (temperature + flux) -----
+    Ni = 256
+    xi, rRi = sample_interface_Ri(Ni)
+    Th_i, Tw_i, _ = net(xi, rRi)
+    # fluxes: -k dT/dr. With nondim, use ratios; set k_w/k_h = kw_rel, etc.
+    dTh_dr = grads(Th_i, rRi)
+    dTw_dr = grads(Tw_i, rRi)
+    kw_over_kh = 1.0
+    L_int_Ri = (Th_i - Tw_i).pow(2).mean() + (dTh_dr - kw_over_kh * dTw_dr).pow(
+        2
+    ).mean()
+
+    xo, rRo = sample_interface_Ro(Ni)
+    _, Tw_o, Tc_o = net(xo, rRo)
+    dTw_dr_o = grads(Tw_o, rRo)
+    dTc_dr_o = grads(Tc_o, rRo)
+    kc_over_kw = 1.0
+    L_int_Ro = (Tc_o - Tw_o).pow(2).mean() + (kc_over_kw * dTc_dr_o - dTw_dr_o).pow(
+        2
+    ).mean()
+
+    # ----- Boundary conditions -----
+    Nb = 256
+    x_in_h, r_in_h = sample_inlet_hot(Nb)
+    Th_in_pred = net(x_in_h, r_in_h)[0]
+    L_in_h = (Th_in_pred - T_h_in).pow(2).mean()
+
+    x_in_c, r_in_c = sample_inlet_cold_counterflow(Nb)
+    Tc_in_pred = net(x_in_c, r_in_c)[2]
+    L_in_c = (Tc_in_pred - T_c_in).pow(2).mean()
+
+    # Outlets: convective (∂T/∂x ≈ 0)
+    x_out_h, r_out_h = sample_outlet_hot(Nb)
+    Th_out = net(x_out_h, r_out_h)[0]
+    L_out_h = grads(Th_out, x_out_h).pow(2).mean()
+
+    x_out_c, r_out_c = sample_outlet_cold(Nb)
+    Tc_out = net(x_out_c, r_out_c)[2]
+    L_out_c = grads(Tc_out, x_out_c).pow(2).mean()
+
+    # Symmetry at r=0: dTh/dr = 0
+    Ns = 128
+    xs = torch.rand(Ns, 1, device=device, dtype=dtype, requires_grad=True)
+    rs0 = torch.zeros_like(xs, requires_grad=True)
+    Th_axis = net(xs, rs0)[0]
+    L_sym = grads(Th_axis, rs0).pow(2).mean()
+
+    # ----- Total loss with simple weights (tune these!) -----
+    L = (
+        1.0 * L_pde
+        + 5.0 * (L_int_Ri + L_int_Ro)
+        + 2.0 * (L_in_h + L_in_c)
+        + 1.0 * (L_out_h + L_out_c)
+        + 1.0 * L_sym
+    )
+
+    L.backward()
+    opt.step()
+
+    if it % 1000 == 0:
+        print(
+            f"it={it}  L={L.item():.3e}  PDE={L_pde.item():.3e}  IF={L_int_Ri.item()+L_int_Ro.item():.3e}"
+        )
diff --git a/Zettelkasten/Fleeting Notes/Journal/2025_08_10.md b/Zettelkasten/Fleeting Notes/Journal/2025_08_11.md
similarity index 90%
rename from Zettelkasten/Fleeting Notes/Journal/2025_08_10.md
rename to Zettelkasten/Fleeting Notes/Journal/2025_08_11.md
index 8c23c2ec..d69d2789 100644
--- a/Zettelkasten/Fleeting Notes/Journal/2025_08_10.md	
+++ b/Zettelkasten/Fleeting Notes/Journal/2025_08_11.md	
@@ -1,7 +1,7 @@
 ---
 ---
 
-# 2025-08-10
+# 2025-08-11
 Today I'm finishing the thesis ideas, for real this time.
 
 I'm also going to get a model of a heat exchanger working.
diff --git a/Zettelkasten/Fleeting Notes/Journal/2025_08_12.md b/Zettelkasten/Fleeting Notes/Journal/2025_08_12.md
new file mode 100644
index 00000000..02d0d774
--- /dev/null
+++ b/Zettelkasten/Fleeting Notes/Journal/2025_08_12.md	
@@ -0,0 +1,10 @@
+---
+---
+
+# 2025-08-12
+
+Yesterday I finished my thesis ideas! Hooray! It feels
+really good to actually have some writing finished. Today,
+I'm making a model for a simple heat exchanger in python.
+Then, I'm going to train a physics informed neural network
+on it and start fucking with parameters.